This invention relates to an air-cleaning device for reducing aerosol concentrations in a confined space such as a factory, shed, greenhouse, hall, shopping mall or room.
High aerosol concentrations can pose a health hazard through breathing the suspended particles.
In farming high aerosol concentrations are found in situations such as poultry sheds and intensive pig rearing sheds etc., the health of both workers and animals is at risk.
In industry a variety of processes such as welding, grinding, smelting and use of internal combustion engines in confined spaces all produce high polluting aerosol concentrations in enclosed spaces.
In social and domestic situations, aerosol pollution is produced by tobacco smoking. Sneezing can produce aerosols of bacteria and viruses. Allergy producing pollen is found in high concentrations at various times of the year. Dust mite allergen particles are produced when making up beds and enter the air as an aerosol.
Conventional air cleaners remove particles from the air by trapping them either in filters (filtration air cleaners (FAC""s)) or by collecting them on plates (electrostatic precipitation air cleaners (ESPAC""s)). The filters or plates may then be disposed of, washed or replaced.
U.S. Pat No. 4234324 discloses an electrostatic air filter comprising closely spaced planar electrodes of conductive material separated by corrugated spacers at edges thereof.
The disadvantages associated with FAC""s are:
1. The efficiency of the filter often drops off markedly with time.
2. The pressure drop across the filter is often high and so requires a powerful fan.
3. The powerful fans are often noisy and consume considerable power.
4. The filters need to be regularly replaced.
The advantages associated with ESPAC""s are:
1. Lower pressure drop.
2. Low noise and low power.
3. Washable collection plates.
The disadvantages associated with ESPAC""s are:
1. Costly shielding of the high voltage metal collecting plates. The user needs to be protected from the possibility of electrical shock from the high voltage power supply (typically several kilovolts). Even when the power supply is switched off, there is danger of shock from stored electrical charge on the plates. The plates need to be removed for cleaning and so a safety interlock is usually provided to automatically discharge the plates before gaining access to them.
2. Loss of efficiency and generation of ozone caused by electrical breakdown and leakage between the metal plates.
3. The plates need to be relatively widely spaced to reduce electrical breakdown in the air between the plates. This reduces efficiency.
An object of the present invention is to provide a practical device for use in removing particles from an air or gas stream substantially without the disadvantages associated with ESPAC""s.
According to a first aspect of the invention there is provided a particle precipitation device for removing particles entrained in a gas stream comprising an array of passages through which the gas stream can pass relatively freely, the passages being provided enclosed by plastics walls, means for urging the gas stream through the array, the plastics walls having areas of conductive material in contact therewith and externally of the passages, and means for applying high and low electrical potentials alternately to isolated areas of the conductive material to provide charged sites in the array for collecting particles from the gas stream.
According to a second aspect this invention provides a particle precipitation device for removing particles entrained in a gas stream comprising an array of passages through which the gas stream can pass relatively freely, the passages being enclosed by plastics walls having electret properties, means for urging the gas stream through the array, whereby particles are collected from the gas stream in the passages.
The passages are preferably provided by fluted plastics sheet preferably having conductive material on opposite external faces thereof. The fluted plastics sheets may, for example, be overlaid one on top of the other, folded in concertina fashion, formed into a spiral, or in a concentric array.
Alternatively, the passages may be provided by plastics tubes arranged side by side. The plastics tubes may be of rectangular cross-section or of circular cross-section.
Yet again the passages may be formed between walls of corrugated plastics sheet or between flat plastics sheets and corrugated conductive material.
The plastics material used in the invention is preferably of polypropylene, polyethylene or a copolymer thereof. Although other plastics materials such as PVC, PET, PTFE and polycarbonate may also be suitable.
For embodiments of the first aspect of the invention the areas of conductive material are preferably of high impedance material but may be of low impedance material. Alternate plastics sheets may have respectively areas of high impedance material and low impedance material thereon.
The high impedance material is preferably cellulose based material, such as paper. Alternative high impedance materials include paint or ink or anti-static coatings.
The low impedance material may be selected from metal sheet, metal film, carbon based films and carbon based paints.
The conductive material is preferably spaced inwardly from edges of the plastics walls except where connection is made to the means for applying potential thereto.
Preferred embodiments of the invention further comprise means for electrically charging particles in the gas stream prior to the array of passages. Such means may be corona discharge means or radioactive ionisation means.
Preferred embodiments of the first aspect of the invention comprise alternate layers of fluted plastics sheet conductive material at high and low electrical potentials, wherein the conductive material is spaced inwardly from edges of the plastics sheets to induce leakage of high voltage and hence ion leakage for charging particles entering the device. The areas of low electrical potential are preferably at ground potential.
High impedance material used in the invention preferably has a thin film resistivity in the range of 109 to 1011 ohms per square.
Devices of the first aspect of the invention preferably comprise a high voltage power supply for powering the high electrical potential areas and a connection lead between the power supply and those areas made of insulated high impedance material.
Devices of preferred embodiments of the invention further comprise means for ionising the gas stream as it leaves the array. The means for ionising the gas steam as it leaves the array preferably comprises a primary corona discharge emitter and a secondary corona discharge emitter at a lower potential to the primary emitter. The primary emitter is preferably connected to high negative potential whilst the secondary emitter is preferably earthed. The primary emitter is preferably a needle having a sharp tip and the secondary emitter is preferably a needle having a relatively blunt tip.
In preferred embodiments of the second aspect of this invention the plastics walls are electrically charged prior to inclusion in the device. The plastics walls may be charged by means of electrodes applied to opposite sides of the walls with a high voltage difference applied thereto. Alternatively, the plastics walls may be charged by applying an electric field at a higher temperature and then cooling to a lower temperature in the presence of the electric field. The plastics walls may also be charged by moving the plastics walls between a high potential corona discharge on one side and an earthed conductive plate on the other side.
In another preferred embodiment of the second aspect of the invention the plastics walls may be provided by faces of fluted plastics sheet material and charging may be by means of filling the flutes with a conductive liquid connecting the flute insides to ground potential and outer faces of the sheet material to high negative and positive potentials respectively.
Another means of charging the plastics walls may be by feeding them between rollers of conductive or semi-conductive material maintained at high and low electrical potentials respectively.
It is also preferred that opposed sides of the walls are rendered conductive and electrically connected together. The plastics walls may be rendered conductive by application of a conductive coating or a conductive sheet material.
Devices of the invention generally comprise a series of spaced plates that are alternately at high and low electrical potential. The high potential plates are electrically isolated from the low potential plates. The high potential plates may be positive or negative with respect to the low potential plates. The low potential plates may form a linear spaced array of plates or a circular spaced array of plates or a spiral spaced array of plates or other conveniently spaced array. The high potential plates are fabricated from a special high impedance material and not from metal (which is a low impedance material (LIM)). The high impedance material (HIM) of the high potential plates allows the plates to rise to their full working electrical potential but disallows them being a shock hazard. When the high impedance (HIM) high potential plates are touched by a person e.g. the user, the current flow is restricted to a low value which causes no shock and no hazard to health. As a result the series of spaced collector plates need no longer be hidden for protection within the air cleaner, but instead can if required be mounted externally for easy access and removal for washing of the plates.
The high potential plates need to be powered from a high voltage power supply. According to the present invention there is also provided a special lead for the purpose of connection to the high potential plates made of high impedance material (HIM). The HIM lead would be insulated with a plastic in the conventional manner, but if the insulation was breached the lead would not present a shock hazard due to the limitation of low current flow from within the lead.
Air entering the series of spaced plates is blown or drawn through the array of plates typically by use of an electrically driven fan. As they pass through the plates, the charged particles (positively or negatively charged) and any electrically neutral particles are subject to a strong electrical field which results in their being drawn to and collected on the plates. The plates can be designed to be either disposable or washable.
In one preferred embodiment both the high potential and low potential sets of plates are made of HIM.
In another preferred embodiment of the invention, the high potential HIM plate is covered with an insulation film.
In yet another preferred embodiment of the invention, both the high potential HIM plates and the low potential plates are covered with an insulation film.
In an alternative preferred embodiment, the gap between the high potential and low potential plates is occupied by an insulating plastics twin-wall fluted sheet material through which air passes.
In another preferred embodiment, the high potential and low potential plates sandwiching the insulating plastics twin-wall fluted sheet material are initially connected to the high voltage power supply and then disconnected.
Particle collection devices of the invention may be based on an electret which is a piece of dielectric material exhibiting a long-lasting electric charge. The electret charge may consist of surface charge layers, charges within the dielectric, polarisation charges or combinations of these.
A thin film electret exhibits an external electrostatic field if its polarisation and space charges do not compensate each other everywhere in the dielectric. This external electrostatic field is utilised in air cleaning filter material manufactured from thin film polymer electret. The thin film polymer is electrically charged to produce a non-woven filter fabric. When air containing suspended particles is passed through the fabric the particles are subjected to strong electrostatic fields as they approach the electret fibres. These forces result in deposition of the particles on the fibres. This fibrous electret polymer filter material has an advantage over conventional fibrous filter media (such as microfine glass fibres) in that high efficiencies can be achieved at relatively low pressure drops.
However, there is a further requirement for a filter medium which can provide high efficiency at even lower pressure drops.
Plastics sheet materials, especially plastics twin-wall fluted plastics sheet material, may be pre-treated to give it electret properties, and that material used in an air-cleaning collection device. Plastics materials suitable for the manufacture of the sheet materials include polyethylene (PE), polypropylene (PP), co-polymers of ethylene and propylene, PVC, PET, PTFE, polycarbonate and others. The plastics materials used preferably provide passages through which air passes readily through the flutes and so the pressure drop through such an air-cleaning array is small. Particles in the air stream passing through are subject to strong electric fields within the passages. Charged particles move in the electric field (by a process termed electrophoresis) toward the passage walls where they adhere and are thus captured.
Because the electric field in the passages is non-linear, uncharged or neutral particles also move (by a process termed dielectrophoresis) toward the walls and are captured.
Whereas most electret air cleaning materials are manufactured to exhibit external electric fields on the surface of polymer films, in this invention care is taken to maximise the electric field strengths inside the air space within the passages of the plastics material.